中文摘要：

2011年12月至2012年1月长江口的现场观测资料表明,大潮和大潮后中潮期间,北港的淡水向北支口门和下段扩展,减小了北支下段的盐度,而小潮及其后的中潮期间无此现象。为了确证淡水的来源,采用验证良好的三维数值模式再现了上述观测现象,并通过数值试验和淡水通量机制分解方法分析其动力成因。数值试验结果表明,大潮及其后的中潮期间,北港主槽流经北港北汊及其周围浅滩的水体是北支淡水的主要来源,拉格朗日余流输送是其向北支扩展的主要动力机制。北港北汊的增深增加了进入北支的淡水通量,减弱了北支的盐水入侵。偏北风风速的增加抑制了北港淡水向北支的扩展,增加了北支的盐水入侵。

英文摘要：

According to the field data collected in the Changjiang Estuary in December 2011 and January 2012,there was a freshwater extension from the North Channel（ NC） to the mouth and lower reaches of the North Branch（ NB）where the water salinity was decreased during spring and the subsequent middle tides. However,the phenomenon was absent during the neap tides and subsequent middle tides. To reveal the origin of the freshwater in the lower reaches of the NB,We employed the well validated 3-D numerical model to reproduce the observed phenomenon. The numerical experiments were conducted and the freshwater flux decomposing method was used to investigate the underlying dynamic mechanism of the freshwater extension. We found that the freshwater in the NC extends to the NB mainly through its northern outlet（ NONC） and neighboring shoals during spring and the subsequent middle tides due to the Lagrangian transports. Deepening of NONC facilitated the freshwater extension,and hindered the saltwater intrusion into the NB. In the contrast,the high-speed northerly winds would restrict the freshwater extension,while assist the saltwater intrusion into the NB.